Sky science
Celestial Horizon
Direct answer
Celestial Horizon: The Celestial Horizon is the great circle 90 degrees from the zenith that divides the ideal visible and hidden hemispheres.
It is the mathematical horizon of the local sky.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Great Circle, Small Circle, Reference Epoch, J2000 Epoch
It is the mathematical horizon of the local sky.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind Celestial Horizon
Horizon geometry has guided observing and navigation since antiquity.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Celestial Horizon matters
Celestial Horizon anchors altitude measurements and theoretical rise and set calculations.
Celestial Horizon in practice: an example
An object at geometric altitude zero lies on the celestial horizon.
Where readings of Celestial Horizon go wrong
Mountains, buildings, refraction, and observer height change practical visibility.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Astronomical Almanac Glossary - Definitions for astronomical coordinates, time, reference systems, and observable phenomena.
- NASA Science: Skywatching FAQ - NASA guidance for observing stars, planets, constellations, and changing sky events.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
Separate the ideal and visible horizon
A definition tells you what it is; your own birth chart tells you what it means for you. The Oracle casts your real chart and answers for your love, your work, and your year ahead, in conversation. It opens with the 7-day Seeker trial.
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More in Astronomy & Skywatching
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Common questions
What is the Celestial Horizon?
The Celestial Horizon is the great circle 90 degrees from the zenith that divides the ideal visible and hidden hemispheres.
What do people get wrong about the Celestial Horizon?
Mountains, buildings, refraction, and observer height change practical visibility.
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